Device and method for monitoring a capacitor bushing

ABSTRACT

The invention relates to a method for monitoring a capacitor bushing ( 1 ) to which an electrical operating voltage (UB) is applied and in which a voltage divider is formed with an electrically conductive insert ( 4 ), whereby at least one measured value (UM1) of an electrical measured quantity (UM) is recorded and stored by using a measuring tap ( 7 ), which is connected to the insert ( 4 ), and by using earth potential. The aim of the invention is to improve the method so that it is only slightly influenced by changes in the operating voltage. To this end, the impedance (ZE) between the measuring tap ( 7 ) and the earth potential is modified after recording the at least one measured value (UM1), and at least one signal value (US1) of a measurement signal (US) subsequently formed is recorded and stored using the measuring tap ( 7 ) and the earth potential. The temporal interval between the time of recording of the measured value (UM1) and the time of recording of the signal value (US1) is measured in such a manner that a change in the operating voltage (UB), which potentially occurs between both times, is negligible. The invention also relates to a device for monitoring a capacitor bushing.

[0001] The invention relates to a method for monitoring a capacitorbushing to which an electrical operating voltage is applied and in whicha voltage divider is formed with an electrically conducting insert, atleast one measured value of an electrical measured quantity beingrecorded and stored by using a measuring tap, which is connected to theinsert, and by using ground potential.

[0002] Such a method is known from EP 0 829 015 B1; it serves forrecording dangerous changes in the dielectric strength of theinsulation. In the case of the known method, only the partial voltage ofa capacitive divider present between the measuring tap and groundpotential is recorded as the electrical measured quantity and monitoredfor a change in the partial voltage. In this case, obviously a number ofmeasured values, for example the amplitude of the partial voltagerespectively at successive points in time, are recorded and stored. Inthe case of the known method, the temporal interval between twosuccessive detected changes in the partial voltage is ascertained andthe frequency of the changes occurring per unit of time is used as abasis for concluding the insulating state of the capacitor bushing. Inthe case of the known method, the point in time of a change must berecorded as accurately as possible. This requires great measurementeffort, since for this purpose the measured values must be ascertainedat temporally very short intervals. What is more, the deviation of theoperating voltage from a nominal value has an effect on the accuracy ofthe evaluation, since a deviation of the operating voltage from itsnominal value also has the consequence of a deviation of thecorresponding partial voltage. This is disadvantageous in particularduring a sustained change in the operating voltage.

[0003] It is therefore an object of the invention to specify a method ofthe type stated at the beginning that is influenced comparatively lessby a change in the operating voltage.

[0004] To achieve this object, the invention provides in the case of amethod of the type stated at the beginning that the impedance betweenthe measuring tap and the ground potential is changed after recordingafter recording the at least one measured value, and at least one signalvalue of a measurement signal then forming is recorded and stored usingthe measuring tap and the ground potential, the temporal intervalbetween the point in time of recording the one measured value and thepoint in time of recording the one signal value being set such thatpossible changes in the operating voltage between the two points in timeare negligible; on the basis of the measured value and the signal value,quotient formation is used to ascertain a characteristic quantity, whichis compared with a predetermined set value and, if the characteristicquantity deviates from the predetermined set value, a status signalindicating a fault of the capacitor bushing is formed.

[0005] The characteristic quantity ascertained by quotient formation isvirtually independent of the level of the operating voltage and also offluctuations in the operating voltage. It is therefore not necessary toknow the exact value of the operating voltage. All that is important isthat the operating voltage had virtually the same value at both pointsin time—that is at the point in time of ascertaining the measured valueand at the point in time of ascertaining the signal value. Therefore,the characteristic quantity can be advantageously ascertained and usedfor an assessment of the insulation of the capacitor bushing even in thecase of an operating voltage deviating from its nominal value. Thecharacteristic quantity which was ascertained for the still undamaged oruninfluenced capacitor bushing or else a characteristic quantityestimated or calculated for this capacitor bushing may be used here asthe set value.

[0006] A further solution achieving the object stated above as providedby the invention in the case of a method of the type stated at thebeginning is that the impedance between the measuring tap and the groundpotential is changed after recording the at least one measured value,and at least one signal value of a measurement signal then forming isrecorded and stored using the measuring tap and the ground potential,the temporal interval between the point in time of recording the onemeasured value and the point in time of recording the one signal valuebeing set such that possible changes in the operating voltage betweenthe two points in time are negligible; on the basis of the voltagedivider equation respectively applying for the measured value and forthe signal value, the signal value, the measured value, the unchangedimpedance and the changed impedance are used to ascertain the impedancebetween the insert and the conductor of the capacitor bushing to whichthe operating voltage is applied; this impedance is compared with apredetermined set value and, if the impedance deviates from thepredetermined set value, a status signal indicating a fault of thecapacitor bushing is formed.

[0007] The impedance between the insert and the conductor of thecapacitor bushing to which the operating voltage is applied is likewisea quantity which is independent of the level of the operating voltageand on the basis of which the insulating state of the capacitor bushingcan be concluded. In this case, the corresponding impedance in the caseof a still undamaged or unchanged capacitor bushing serves as the setvalue.

[0008] The changing of the impedance between the measuring tap and theground potential is preferably carried out when the measured quantity,or the measurement signal, is as low as possible. As a result, thechanging of the impedance took place, so that loading of a switchingdevice required for this purpose that is caused by a measuring currentis as low as possible.

[0009] It is preferred for the measured value to be recorded in a firsthalf-period of the operating voltage and the signal value to be recordedin the subsequent second half-period of the operating voltage. As aresult, the measured value and the signal value are recorded at a veryshort temporal interval, so that a fluctuation in the operating voltagehas very largely no influence on the monitoring.

[0010] Preferably, a number of characteristic quantities are ascertainedone after the other and an average value of the characteristic quantityis formed. The average value of the characteristic quantity may then beused additionally for comparison with the set value, the average valueof the characteristic quantity advantageously being virtually entirelyindependent of a fluctuation in the operating voltage.

[0011] The impedance lying between the measuring tap and the groundpotential is preferably changed by connecting in or disconnecting aknown fixed impedance. This is a simple possible way of changing theimpedance between the measuring tap and the ground potential.

[0012] The invention also relates to a device for monitoring a capacitorbushing to which an electrical operating voltage is applied, in which avoltage divider is formed with an electrically conducting insert, ameasuring tap connected to the insert being provided and connected to ameasuring device for recording an electrical measured quantity.

[0013] Such a device is also known from EP 0 829 015 B1, already citedabove. The known device is merely connected to the measuring tap of thecapacitor bushing by means of a lead. The partial voltage applied to themeasuring tap at ground potential is used as the electrical measuredquantity.

[0014] It is an object of the invention to specify a device of the typestated at the beginning with which a capacitor bushing can be monitoredlargely uninfluenced by fluctuations in the operating voltage.

[0015] This object is achieved according to the invention by theimpedance present between the measuring tap and ground potentialcontaining an impedance arrangement which is assigned a switchingdevice.

[0016] A control device is preferably provided for activating theswitching device. With the aid of the control device, the switchingdevice can be automatically activated, so that the impedance can beautomatically changed.

[0017] It is further preferred for the measuring device to have aquotient former. With the quotient former, the quotient can beadvantageously formed from a measured value ascertained with theimpedance unchanged and a signal value ascertained with the impedancechanged. This quotient can then be used as a characteristic quantity forthe state of the capacitor bushing.

[0018] It is further preferred for the impedance arrangement to containa capacitor, which on the one hand is connected to ground potential andon the other hand is connected via the switching device to the measuringtap. The capacitor can be connected to the measuring tap anddisconnected from it by means of the switching device.

[0019] It is preferred for the impedance arrangement to have a furtherfixed impedance, which is connected between the measuring tap and theground potential. The further fixed impedance may be chosen such thatthe divider ratio of the voltage divider is influenced in such a waythat the measured quantity and the measurement signal are of orders ofmagnitude which can be measured well.

[0020] The method and the device are explained on the basis of thedrawing with the single figure.

[0021] Represented in the figure is a capacitor bushing 1 with a centralconductor 2, for example a high-voltage conductor, which is surroundedby an insulator 3. Arranged on the insulator 3 is a metallic flange 8for securing the capacitor bushing in a housing wall (not represented).Fixed in the insulator 3 is a conducting insert 4, which is electricallyinsulated from the electrical conductor 2 and surrounds the latter. Thecapacitor bushing 1 may also have a number of such inserts, which forthe sake of clarity however only this one insert 4 is represented. Thisis connected in an electrically conducting manner to a measuring tap 7.The conductor 2 is connected to a high-voltage line 9, to which anoperating voltage UB is applied. The flange 8 is at ground potential.

[0022] The measuring tap 7 is connected via an impedance arrangement 10to a measuring device 11. The impedance arrangement 10 has a fixedimpedance 12, which can be connected to the measuring tap 7 and can bedisconnected from the measuring tap 7 via a switching device 13. Theswitching device 13 is connected to a control device 14. The switchingdevice 13 may be configured for example with a semiconductor switch. Thefixed impedance 12 is configured here by way of example as a capacitor12A. The impedance arrangement 10, the measuring device 11 and thecontrol device 14 form a device 18 for monitoring the capacitor bushing1.

[0023] The conducting insert 4 forms a voltage divider. The oneimpedance of the voltage divider is formed by the impedance ZH, whichlies between the conducting insert 4 and the conductor 2 andsubstantially comprises a capacitance 5 (represented by dashed lines).The other impedance of the voltage divider is formed by the impedanceZE, which lies between the conducting insert 4 and the ground potential.This impedance ZE comprises the capacitance 6 (represented by dashedlines) lying within the capacitor bushing 1 between the insert 4 and theground potential, the impedance arrangement 10 parallel to saidcapacitance and the internal resistance (not represented in any moredetail) of the measuring device 11.

[0024] The capacitor bushing 1 is monitored for a fault by the device18. A fault may be a change in the capacitance 5. Generally, thecapacitance 5 is increased by such a fault. An increase in thecapacitance 5 is synonymous with a reduction in the insulationresistance between the conductor 2 and the insert 5.

[0025] For monitoring the capacitor bushing 1, the impedance arrangement10 is initially in a first measuring state, in which the switchingarrangement 13 is open and the fixed impedance 12 is not connected tothe measuring tap. In this first measuring state, a measured value UM1of an electrical measured quantity UM is recorded at a first point intime t₁ and stored in a memory (not represented in any more detail) inthe measuring device 11. This measured quantity UM is in this case theelectrical voltage applied to the measuring tap at ground potential. Inthis measuring state of the impedance arrangement 10, the impedance ZEis formed by the parallel connection of the capacitance 6 and theinternal resistance (not represented in any more detail) of themeasuring device 11. The impedance in this measuring state is referredto as unchanged impedance ZE1.

[0026] After recording the measured quantity UM1, the impedancearrangement 10 is put into a second measuring state. For this purpose,the control device 14 is put into the closed state under the control ofthe switching device 13. As a result, the fixed impedance 12 is nowconnected in an electrically conducting manner to the measuring tap 7.The impedance ZE is now formed by the parallel connection of thecapacitance 6, the internal resistance (not represented in any moredetail) of the measuring device 11 and the fixed impedance 12. In thissecond measuring state, a signal value US1 of a measurement signal USthen forming is recorded by the measuring device 11 at a second point intime t₂ and likewise stored. The measurement signal US is the electricalvoltage applied to the measuring tap at ground potential. The impedanceZE in this second measuring state is referred to as changed impedanceZE2.

[0027] The temporal interval between the point in time t₁ and the pointin time t₂ is chosen such that a possibly occurring change in theoperating voltage UB between the two points in time t₁ and t₂ isnegligible. This may for example take place so quickly that the changein the operating voltage is negligible on account of its own periodicity(with the system frequency) . It can then be assumed that the amplitudeof the operating voltage UB has remained constant between the two pointsin time t₁ and t₂.

[0028] For the measured value UM1, the following voltage dividerequation G1 applies: $\begin{matrix}{\frac{UM1}{UB} = \frac{ZE1}{{ZE1} + {ZH}}} & \left( \text{G1} \right)\end{matrix}$

[0029] For the signal value US1, the following voltage divider equationG2 applies: $\begin{matrix}{\frac{US1}{UB} = {\frac{ZE2}{{ZE2} + {ZH}}.}} & \left( \text{G2} \right)\end{matrix}$

[0030] Since, as already explained above, it can be presupposed that theoperating voltage UB has remained constant, the operating voltage UB canbe eliminated by equating the correspondingly transformed equations G1and G2, and the following equation G3 applies: $\begin{matrix}{{{UM1}\frac{{ZE1} + {ZH}}{ZE1}} = {{US1}\frac{{ZE2} + {ZH}}{ZE2}}} & \left( \text{G3} \right)\end{matrix}$

[0031] As mentioned above, when there is a fault of the capacitorbushing 1, only the impedance ZH changes, and the capacitance 6 remainsunchanged. Consequently, apart from the impedance ZH, all the quantitiesin the equation G3 are known, or constant; a relationship for acharacteristic quantity K which is solely dependent on the impedance ZHcan be obtained from the equation G3. This characteristic quantity K iscompared with a predetermined set value K0. If the characteristicquantity K deviates from the predetermined set value K0, a status signal16 indicating a fault of the capacitor bushing 1 is formed. Acharacteristic quantity which has been ascertained by the methodaccording to the invention in the case of an unchanged or undamagedcapacitor bushing can be used as the set value K0; the characteristicquantity may, however, also be ascertained when the capacitor bushing 1is put into operation or be determined by calculation.

[0032] The quotient of the measured value UM1 and the signal value US2may be formed as a characteristic quantity K1. For this purpose, aquotient former 15 provided in the measuring device 11 is used. A faultof the capacitor bushing 1, that is a change in the capacitance 5, hasan effect on the divider ratio of the voltage divider and consequentlyon the measured quantity UM and the measurement signal US. However, themeasured quantity UM and the measurement signal US change differently inpercentage terms. On the other hand, when there is a change in theoperating voltage UB, the measured quantity UM and the measurementsignal US change identically in percentage terms. The different changein percentage terms of the measured quantity UM and the measurementsignal US with an unchanged capacitance 5 produces a different quotientwith respect to the set value K0, and consequently a characteristicquantity K deviating from it. The level of the operating voltage UB, onthe other hand, has virtually no effect on the quotient, andconsequently on the characteristic quantity K. In particular, even thecharacteristic quantity K ascertained during a sustained deviation ofthe operating voltage UB from its nominal value can be used for acomparison with the set value K0. In this particular case it is notnecessary to know the operating voltage UB exactly. Therefore, no greatefforts have to be taken to record it. It is likewise not necessary forthe point in time of a change to be recorded with any great precision oreffort.

[0033] The capacitance 6 of the capacitor bushing 1, the internalresistance (not represented in any more detail) of the measuring deviceand the impedance module 12, and consequently the impedances ZE1 andZE2, are generally known. With the impedances ZE1 and ZE2 and also themeasured value UM1 and the signal value US1, it is possible with the aidof the equation G3 to calculate impedance ZH, or the capacitance 6, asthe characteristic quantity KG. After that, the impedance ZH is obtainedaccording to the following equation G4 as $\begin{matrix}{{ZH} = {{{ZE1} \cdot {ZE2}}{\frac{{US1} - {UM1}}{{{UM1} \cdot {ZE2}} - {{US1} \cdot {ZE1}}}.}}} & \text{(G4)}\end{matrix}$

[0034] A particularly simple case is obtained if the impedances ZE1, ZE2and ZH can in each case be considered as pure capacitances. Then themeasured value UM1 and the signal value US1 merely have to beascertained in terms of their amount, but not their phase. As a result,a particularly simple calculation of the characteristic quantity K or KCis obtained.

[0035] To set the orders of magnitude of the measured quantity UM and ofthe measurement signal US respectively to an order of magnitude which iswell suited for measurement, the impedance arrangement 10 may have afurther fixed impedance 17 (represented by dashed lines). Bycorresponding choice of the value of the further fixed impedance 17, thedivider ratio of the voltage divider is correspondingly influenced. Thefixed impedance 17 may be chosen for example as the capacitance. Thefixed impedance 17 must then be taken into account in the aboveequations in the impedances ZE1 and ZE2.

[0036] Since the operating voltage UB is generally periodic, themeasured quantity UM and the measurement signal US are also periodic.The changing of the impedance ZE takes place when the measured quantityUM or the measurement signal US are low. This is the case in theproximity of a zero crossing of the measured quantity UM or themeasurement signal US. As a result, the switching device 13 is switchedin a largely unloaded state. The maximum value of the measured quantityUM in a first half-period of the operating voltage UB can be recorded asmeasured value UM1 and the maximum value of the measurement signal US inthe subsequent second half-period of the operating voltage UB can berecorded as signal value US1. It is also possible to record in ameasuring state a number of maximum values and use them to determine anaveraged maximum value, which is used for the actual calculation of thecharacteristic quantity. The averaging allows noise influences to bereduced.

[0037] With the measuring device 11 and the control device 14 it is alsopossible for a number of characteristic quantities to be ascertained oneafter the other and by corresponding control of the impedancearrangement 10. The measuring device 11 may contain an average-valueformer (not represented in any more detail), with which an average valueMK of these characteristic quantities is formed. The average value MKhas a very low dependence on fluctuations in the operating voltage UB.

[0038] The current between the measuring tap 7 and the impedancearrangement 10, obtained in the first measuring state or the secondmeasuring state, may also be used as the measured quantity UM or as themeasurement signal US.

1. A method for monitoring a capacitor bushing (1) to which anelectrical operating voltage (UB) is applied and in which a voltagedivider is formed with an electrically conducting insert (4), at leastone measured value (UM1) of an electrical measured quantity (UM) beingrecorded and stored by using a measuring tap (7), which is connected tothe insert (4), and by using ground potential, characterized in that theimpedance (ZE) between the measuring tap (7) and the ground potential ischanged after recording the at least one measured value (UM1), and atleast one signal value (US1) of a measurement signal (US) then formingis recorded and stored using the measuring tap (7) and the groundpotential, the temporal interval between the point in time of recordingthe one measured value (UM1) and the point in time of recording the onesignal value (US1) being set such that possible changes in the operatingvoltage (UB) between the two points in time are negligible, on the basisof the measured value (UM1) and the signal value (US), quotientformation is used to ascertain a characteristic quantity (K), which iscompared with a predetermined set value (K0) and, if the characteristicquantity (K) deviates from the predetermined set value (K0), a statussignal (16) indicating a fault of the capacitor bushing is formed.
 2. Amethod for monitoring a capacitor bushing (1) to which an electricaloperating voltage (UB) is applied and in which a voltage divider isformed with an electrically conducting insert (4), at least one measuredvalue (UM1) of an electrical measured quantity (UM) being recorded andstored by using a measuring tap (7), which is connected to the insert(4), and by using ground potential, characterized in that the impedance(ZE) between the measuring tap (7) and the ground potential is changedafter recording the at least one measured value (UM1), and at least onesignal value (US1) of a measurement signal (US) then forming is recordedand stored using the measuring tap (7) and the ground potential, thetemporal interval between the point in time of recording the onemeasured value (UM1) and the point in time of recording the one signalvalue (US1) being set such that possible changes in the operatingvoltage (UB) between the two points in time are negligible, on the basisof the voltage divider equation (G1, G2) respectively applying for themeasured value (UM) and for the signal value (US), the signal value(US), the measured value (UM1), the unchanged impedance (ZE1) and thechanged impedance (ZE2) are used to ascertain the impedance (ZN) betweenthe insert (4) and the conductor (2) of the capacitor bushing (1) towhich the operating voltage is applied, which impedance is compared witha predetermined set value (K0) and if the impedance (ZH) deviates fromthe predetermined set value (K0), a status signal (16) indicating afault of the capacitor bushing (1) is formed.
 3. The method as claimedin one of the preceding claims, characterized in that the changing ofthe impedance (ZE) between the measuring tap (7) and the groundpotential is carried out when the measured quantity (UM) , or themeasurement signal (US1), is as low as possible.
 4. The method asclaimed in claim 3, characterized in that the measured value (UM1) isrecorded in a first half-period of the operating voltage (UB) and thesignal value (US1) is recorded in the subsequent second half-period ofthe operating voltage (UB).
 5. The method as claimed in one of thepreceding claims, characterized in that a number of characteristicquantities are ascertained one after the other and an average value (MK)of the characteristic quantity is formed.
 6. The method as claimed inone of the preceding claims, characterized in that the impedance (ZE)lying between the measuring tap (7) and the ground potential is changedby connecting in or disconnecting a known fixed impedance (12).
 7. Adevice for monitoring a capacitor bushing (1) to which an electricaloperating voltage (UB) is applied, in which a voltage divider is formedwith an electrically conducting insert (5), a measuring tap (7)connected to the insert (5) being provided and connected to a measuringdevice (11) for recording an electrical measured quantity (UM),characterized in that the impedance (ZE) present between the measuringtap (7) and ground potential contains an impedance arrangement (10)which is assigned a switching device (13).
 8. The device as claimed inclaim 7, characterized in that a control device (14) is provided foractivating the switching device (13).
 9. The device as claimed in one ofclaims 7 to 8, characterized in that the measuring device (11) has aquotient former (15).
 10. The device as claimed in one of claims 7 to 9,characterized in that the impedance arrangement (10) contains acapacitor (12A), which on the one hand is connected to ground potentialand on the other hand is connected via the switching device (13) to themeasuring tap (7).
 11. The device as claimed in one of claims 7 to 10,characterized in that the impedance arrangement (1) has a further fixedimpedance (17), which is connected between the measuring tap (7) and theground potential.